1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method, and more particularly to a technique for generating data for completing printing in the same area of a printing medium by moving a printing head a plurality of times relative to the same area of the printing medium.
2. Description of the Related Art
As this kind of technique, a multi-pass printing method is known for completing an image to be printed in the same area of a printing medium by scanning that same area of the printing medium a plurality of times with a printing head. With this technique, it is possible to reduce density unevenness and stripes in an image. However, even though the multi-pass printing method is employed, there is a possibility that a shift of the dot printing position will occur between the plurality printing scans due to fluctuation in the conveyance amount of the printing medium. Such shift causes fluctuation in the dot coverage, and as a result, image defects such as density fluctuation or density unevenness may occur.
In addition, the fluctuation in the conveyance amount of the printing medium mentioned above differs according to the position of the printing medium on the conveying path of the printing medium. For example, when printing over the entire surface of a printing medium without setting margins, or in other words, performing so-called margin-less printing, the printing position is located on the printing medium where printing is performed by the printing head as the printing medium is conveyed along the conveying path by only one pair of conveying rollers that are provided on both the upstream side and downstream side of the printing area. In such a printing position, the conveyance precision is low and it is easy for fluctuation in the conveyance amount to occur. On the other hand, when the printing medium is conveyed in a state of being held by both of a pair of conveying rollers on the upstream side and downstream side, the conveyance precision is high and it is difficult for fluctuation of the conveyance amount to occur. Moreover, it is known that during conveyance, when the printing medium enters or leaves the nip portion of a pair of conveying rollers, there is a relatively large fluctuation in the conveyance amount.
A method is known as a technique for reducing the image defects caused by the shift of the dot printing position due to the above described fluctuation of the conveyance amount in which image data are divided into respective data corresponding to different scans in the stage of multi-valued image data before binarization, and then the respective divided multi-valued image data are independently binarized (Japanese Patent Laid-Open No. 2000-103088). FIG. 13A is a diagram illustrating the arrangement of dots that are printed based on image data that are processed by the method disclosed in Japanese Patent Laid-Open No. 2000-103088. In the figure, the black dots 551 are dots that are printed in a first printing scan, white dots 552 are dots that are printed in a second printing scan and gray dots 553 are dots that are printed by overlapping of dots that are printed in the first printing scan and second printing scan. In other words, the technique disclosed in Japanese Patent Laid-Open No. 2000-103088 is a technique of generating overlapping dots by reducing mutual complementarity or exclusivity of dots that are printed in a plurality of printing scans.
With this dot arrangement, even though a dot group printed in a first printing scan and a dot group printed in a second printing scan become shifted in the main scanning direction or sub scanning direction, there is not much fluctuation in the dot coverage on the printing medium, and as a result, it is possible to reduce the image defects mentioned above. This is because even though portions, where dots that are printed in the first printing scan and dots that are printed in the second printing scan overlap, newly appear, there are portions where two dots that were originally supposed to be printed such that they overlap do not overlap. More specifically, conventionally, by using a mask on normally quantized data, print data are divided, as well as complementarity and exclusivity is given to data that are to be printed in different printing scans. On the other hand, the method disclosed in Japanese Patent Laid-Open No. 2000-103088 is a method in which multi-valued data are divided in the multi-valued data stage into a plurality of multi-valued data that correspond to different printing scans, and the plurality of multi-valued data are each independently quantized so that quantized data to be used in the respective printing scans are obtained. This allows complementarity between dots that are printed in different printing scans to be decreased, and overlapping dots are caused to occur among the dots that are printed in a plurality of printing scans.
However, in the method disclosed in Japanese Patent Laid-Open No. 2000-103088 it is not possible to control the overlap amount of dots that are printed in a plurality of printing scans. As a result, the number of overlapping dots may become excessive and the graininess of the image may be increased, or conversely, the number of overlapping dots could be too few and the aforementioned density fluctuation may not be sufficiently suppressed.
The inventors of the present application placing attention on making it possible to control the amount of the dot overlap by generating data that is reflected in all the quantized data of the different printing scans in common with each other.
Meanwhile, the degree of graininess or the degree of density unevenness (density fluctuation), which appear in a printed image differs depending on the gradation values for each pixel in the image, or in other words, depending on the dot duty, which is the percentage of the number of dots that are formed with respect to a certain surface area. More specifically, how much dot overlapping is adequate for suppressing density unevenness due to the above-mentioned conveyance error when conveying printing medium differs depending on the dot duty. In addition, this dot overlap amount also affects graininess.
When the degree of density unevenness and graininess that appear in a printed image differs depending on the duty as described above, applying a process of generating data that commonly reflects on the entire printed data is not preferred. In other words, when applying a process of generating data that commonly reflects over the entire printed data, there is a possibility that density unevenness and graininess will become worse. Moreover, performing processes of dividing and quantizing multi-valued image data when generating data having duty for which the processes of dividing and quantizing is not necessary, there is a possibility that the processing load will increase or that the processing speed will decrease.